The present invention relates generally to aircraft gas turbine engines with thrust augmenting afterburners and, more specifically, afterburners with trapped vortex cavities.
High performance military aircraft typically include a turbofan gas turbine engine having an afterburner or augmentor for providing additional thrust when desired particularly for supersonic flight. The turbofan engine includes in downstream serial flow communication, a multistage fan, a multistage compressor, a combustor, a high pressure turbine powering the compressor, and a low pressure turbine powering the fan. A bypass duct surrounds and allows a portion of the fan air to bypass the multistage compressor, combustor, high pressure, and low pressure turbine.
During operation, air is compressed in turn through the fan and compressor and mixed with fuel in the combustor and ignited for generating hot combustion gases which flow downstream through the turbine stages which extract energy therefrom. The hot core gases are then discharged into an exhaust section of the engine which includes an afterburner from which they are discharged from the engine through a variable area exhaust nozzle.
Afterburners are located in exhaust sections of engines which includes an exhaust casing and an exhaust liner circumscribing a combustion zone. Fuel injectors (such as spraybars) and flameholders are mounted between the turbines and the exhaust liner for injecting additional fuel when desired during reheat operation for burning in the afterburner for producing additional thrust. Thrust augmentation or reheat using such fuel injection is referred to as wet operation while operating dry refers to not using the thrust augmentation. The annular bypass duct extends from the fan to the afterburner for bypassing a portion of the fan air around the core engine to the afterburner. This bypass air is mixed with the core gases and fuel from the spraybars prior and ignited and combusted prior to discharge through the exhaust nozzle. The bypass air is also used in part for cooling the exhaust liner.
Various types of flameholders are known and provide local low velocity recirculation and stagnation regions therebehind, in regions of otherwise high velocity core gases, for sustaining and stabilizing combustion during reheat operation. Since the core gases are the product of combustion in the core engine, they are initially hot, and are further heated when burned with the bypass air and additional fuel during reheat operation. Augmentors currently are used to maximize thrust increases and tend to be full stream and consume all available oxygen in the combustion process yielding high augmentation ratios for example about 70%.
A trapped vortex cavity flame stabilizer was developed for afterburners to eliminate the spraybars and flameholders and to stabalize the flame in the afterburner during afterburner operation. The one piece ring trapped vortex cavity acts as a flame stabilizer and is a one piece 360 degree ring structure with radial walls. It is subject to high stresses due to thermal temperature gradients in the radial and circumferential directions and tests have been made and signs of thermal distress have been observed. The ring structure requires thousands of small cooling holes that cannot be easily drilled in a one piece ring structure. The one piece ring trapped vortex cavity is coated with a thermal barrier coating (TBC) which cannot easily be sprayed due to the physical size of the spray nozzle relative to the width and depth of the cavity.
Thus, it is highly desirable to have a trapped vortex cavity for use in an afterburner which has better performance characteristics than previous augmentors and able to withstand the high temperature environment in the afterburner.